Reduction of acidity in synthetic ester lubes with ethylene carbonate



United States Patent REDUCTION OF ACIDITY IN SYNTHETIC ESTER LUBES WITHETHYLENE CARBONATE David W. Young, Westfield, and Delmer L. Cottle,Highland Park, N. J., assignors to Esso Research and EngiieeringCompany, a corporation of Delaware No Drawing. Application October 6,1951, Serial No. 250,184

8 Claims. (Cl. 260-485) This invention relates to complex syntheticester lubricating oils. Particularly this invention relates to a processfor neutralizing synthetic lubricating oil of the complex ester typeusing ethylene carbonate as the neutralizing agent.

In an elfort to obtain superior lubricating oils having specific andunusual characteristics, new synthetic lubricants have been developed.One class of material which has attracted unusual interest as syntheticlubricants are the complex esters. In general these lubricating oils arecharacterized by high viscosity indices and lower pour points thanmineral oils of corresponding viscosity. Lubricants possessing suchproperties are of special value in the lubricating of engines which aresubjected to high temperatures such as combustion turbine engines,particularly those of the prop-jet type. Mineral oil lubricants aregenerally less desirable for use in such engines because of theirtendency to leave a higher percentage of carbonaceous residue whichaccumulates and interferes with the operation of the engine. In additionthe relatively high pour points and low flash points of suchcompositions are undesirable. The synthetic lubricants of the complexester type are especially adaptable to use under such conditions sincethese lubricants may contain no additive and have a desirablecombination of high flash points, low pour points, and high viscosities.

In the preparation of these complex ester synthetic lubricating oils,manufacturers are faced with the problem of lowering the final acidityof the oil to a point where the synthetic lubricant will be acceptablefor use in engines at high temperatures. In the past this residualacidity has been reduced by washing with aqueous alkali or bypercolating through beds of alkali or alkali impregnated on less activematerials. Washing with alkalis', however, frequently results in theformation of stable emulsions and washing alone frequently does notreduce the acidity to a point low enough to be practical. Although thepercolation method may reduce the acidity to the desired point,ester-soluble alkali salts of organic acids apparently result since theester lubricants so treated have been found to possess a high andundesirable ash content.

It has now been found, and forms the object of this invention, that theacidity of these complex ester synthetic lubricating oils may be reducedto the desired point by treatment with ethylene carbonate. In additionto being extremely effective, the process of this invention has thefurther advantage of introducing no ash into the synthetic lubricant orcontaminating the oil in any way because of the presence of inorganicmaterials. As a result the complex ester synthetic lubricant oilswhichhave been treated according to the process of this invention neednot be subsequently washed or filtered before stripping to the desiredviscosity or flash point. Moreover, the amount of ethylene carbonateintroduced into the synthetic lubricating oil is so small that thephysical properties of the final lubricating oil such as viscosity,flash point, fire point and extreme pressure properties are not altered.

The synthetic ester lubricating oils operable in the con "ice cept ofthis invention are those complex esters, distillable or not distillable,which are formed by the interaction of two or more of the followingcompounds in which at least onepolyfunctional alcohol and onepolyfunctional acid are employed:

(a) Monohydric alcohols (11) Monobasic acids (0) Dibasic acids (d)Glycols (e) Polyhydric alcohols (f) Polybasic acids (g) Hydroxy acidsThe synthetic lubricating oils of the complex ester type contemplated bythis inventionare grouped'under one "of four types as follows:

I. M onobasic acid-glycol-dibasic acid-glycol-monobasic acid Thiscomplex ester may be represented by the following structural formula:

R1 and R5 are the alkyl radicals of the monobasic acids, R2 and R4arethe alkyl radicals from the glycols, and R3 is the alkyl radical fromthe dibasic acid.

The esters of this type may be prepared by admixing the calculatedamounts of the various compounds and carrying out a straightforwardesterification reaction. The reaction conditions are continued with anoccasional sample of the product being tested for acidity until theminimum acidity is attained.

II. Alcohol-dibasic acid-glycol-dibasic acid-alcohol This material maybe represented by the following formula:

R1 and Rsare the combining radicals of the alcohol, R2 and R4 are thealkyl radicals of the-dibasic acids, and R3 is the alkyl radical of theglycol.

These esters are prepared in the manner similar to those of I.

III. Alcohol-dibasic acid-glycol-monobasic acid These esters areprepared by reacting a dibasic acid and a glycol under such conditionsthat one hydroxyl group of the glycol combines with one carboxyl groupof the dibasic acid, in other words, a half ester is formed. This halfester is then reacted with atmolar proportion each of an aliphaticalcohol and a monobasic acid. These materials may be said to have thegeneral formula:

R1 is the combining radical of the aliphatic alcohol, R2 the alkylradical of the dibasic acid,

R3 the alkyl radical of the'glycol, and

R4 the alkyl radicals of the monobasic acid.

The preparation of these ester materials is specifically set out incopending application Serial No. 52,428, now Patent No. 2,575,195.

IV. Alc0lz0l-dibasic acid-glycol-dibasic acid-alcohol These materialsmay be said to have the general formula wherein:

R1 and R are the combining alkyl radicals of the alcohol, R2 and R4 thealkyl radicals of the dibasic acid, and Rs'is the alkyl radical of theglycol.

It will be noted that the esters of IV have the same structural formulaas 11. However, these complex esters are prepared by reacting analcoholwith a dibasic acid under such conditions that a half ester isformed and reacting two mols of such an ester with one mol of a glycol.The preparation of this type of synthetic ester lubricating oil is setout in detail in copendin application Serial No. 52,429, now Patent No.2,703,811.

V. Monobasic acid-glycol-dibasic acid-glycol-monobasic acid Thesesynthetic esters may be said to have the general formula:

wherein:

R1 and R5 are the alkyl radicals of the monobasic acid, R2 and R4 arethe alkyl radicals of the glycol, and

R3 is the alkyl radical of the dibasic acid.

It will be noted that these synthetic esters are the same as thoseappearing above under 1 except that this type is prepared by reacting amonobasic acid with a glycol under such conditions that a half ester isformed and reacting two mols of such ester with one mol of a dibasicacid. The details of the preparation of this type of synthetic ester areset out in copending application Serial No. 52,430, now Patent No.2,575,196.

As is stated above, at various stages in the preparation of the complexesters samples are removed from the reaction vessel and are titratedwith a standardized basic solution to determine the acidity. It has beendetermined that the minimum allowable residual acidity in the ester oilsis in the vicinity of 0.1 mg. of potassium hydroxide/ g. In thepreparation of these synthetic esters, while it is theoreticallypossible to completely neutralize the acids, as a practical matter it isfound impossible to lower the acidity to below 1.5 mg. of potassiumhydroxide per m-v As was set out above, these complex ester materialsare prepared by a combination of an alcohol, a monobasic acid, a dibasicacid, and a glycol in various fashions.

Operable alcohols include the following:

Methyl alcohol Ethyl alcohol n-Butyl alcohol n-Hexyl alcohol n-Octylalcohol 2-ethylhexyl alcohol Cetyl alcohol Oleyl alcohol Ethylene glycolmono-n-butyl ether Ethylene glycol mono-2-ethylbutyl ether Ethyleneglycol mono-Z-ethylhexyl ether Ethylene glycol mono-tert.-octyl etherfi-n-Butylmercaptoethanol fi-tert.-octylmercaptoethanolfl-n-Dodecylmercaptoethanol Diethylene glycol mono-n-butyl etherDiethylene glycol mono-Z-ethylbutyl ether Diethylene glycolmono-2-ethylhexyl ether Propylene glycol mono-butyl thioether Propyleneglycol mono-tert.-octyl thioether Propylene glycol mono-n-dodecylthioether n-Butylmercaptoethoxyethanol Tert.-octylmercaptoethoxyethanoln-Dodecylmercaptoethoxyethanol n-ButylmercaptopropoxypropanolTert.-octylmercaptopropoxypropanol n-DodecylmercaptopropoxypropanolPropylene glycol mono-n-butyl ether vDipropylene glycol monomethyl etherDipropylene glycol monoethyl ether Dipropylene glycol mono-n-butyl etherTripropylene glycol monomethyl ether Tripropylene glycol monoethyl etherTripropylene glycol mono-n-butyl ether Propylene glycol monoisopropylether Dipropylene glycol monoisopropyl ether Tripropylene glycolmonoisopropyl ether h lany of the above listed ether alcohols, formed bythe reaction of ethylene oxide or propylene oxide with aliphaticalcohols, are known in the industry as Dowanols, Carbitols, orCellosolves.

A group of alcohols especially adapted for use in connection with thepresent invention are the so-called 0x0 alcohols, prepared by thereaction of carbon monoxide and hydrogen upon the olefins obtainablefrom petroleum products. Materials such as diisobuylene and C7 olefinsare suitable for this purpose; also higher and lower molecular weightolefinic materials are sometimes employed. The alcohols obtained in thismanner normally have a branched chain structure.

Among the rnonobasic acids which may be employed in the preparation ofthe esters of the present invention, the following may be listed asillustrative:

Illustrative examples of the dibasic acids which may be employed in thesynthesis of the complex esters of the present invention are thefollowing:

Oxalic acid Malonic acid Succinic acid Glutaric acid Adipic acid Pimelicacid Suberic acid Azelaic acid Sebacic acid Brassylic acidPentadecanedicarboxylic acid Tetracosanedicarboxylic acid C4-C2;alkenylsuccinic acids Diglycolic acid Thiodiglycolic acid Carbon-alkylderivatives of the above acids as e. g.

2-methyl adipic acid.

The C4-C2; alkenylsuccinic acids listed above are prepared by condensingolefins or mixtures of olefins With maleic anhydride.

The glycols employed in preparing the esters of the present inventioninclude ethylene glycol and any of the parafiinic homologues of the samecontaining up to 18 carbon atoms. These may include, for example,ethylene glycol, propylene glycol, butylene glycols, pinacone,trimethylene glycol, tetramethylene glycol, pentamethylene glycol, andthe like. Since the glycols may also contain oxygen or sulfur. atoms,compounds such as diethylene glycol, triethylene glycol, thepolyethylene glycols of the formula HO CHzCHzO) nCHZCHZOH where n is 1to 26, and the polypropylene glycols of the general formula where eitherR1 or R2 is a methyl group and the other is hydrogen, and where n is 1to 20, may likewise be employed. Glycols containing sulfur atoms inthioether linkages may also be employed, and these include suchcompounds as thiodiglycol and 1,2-bis(2-hydroxyethylmercapto) ethane.There also may be used glycols containing both oxygen and sulfur insimilar linkages; such a compound is bis-2-(2-hydroxyethoxy) ethylsulfide.

In carrying out the process of this invention these complex esters areplaced in suitable reaction equipment and the calculated amount ofethylene carbonate is placed therein. The temperature is raised to thedesired point and the reaction allowed to proceed until the minimumacidity is attained.

The reaction probably proceeds through the formation of an intermediatealkyl carbonate as follows:

The amounts of the ethylene carbonate used to neutralize the residualacidity in the complex ester lubricating oils will depend, of course,upon the final residual acidity of the ester. It has been found thatamounts varying between about 0.5 and 20% by Weight based on the weightof the ester material are sufficient in most instances. Amounts in therange of from 4 to 10% are especially preferred. Unreacted carbonate maybe recovered and recycled to the next batch.

The temperature at which the reaction best occurs will lie within therange of from 100 to 300 C., preferably 125 to 200 C. The time of thereaction is a function of both the residual acidity of the syntheticester and the temperature at which the reaction is carried out. It isnecessary, therefore, to maintain the reaction until such time as thedesired residual acidity is attained. For temperatures within a range offrom 100 to 200 C. it has been found that from 2 to 6 hours issufficient.

In order to more specifically illustrate the process of the instantinvention the following examples are given. It is to be understood thatthese examples are illustrative only and are not considered to belimiting in any way upon the concept of this invention.

Example I 74 g. of adipic acid, 65.6 g. of isooctyl alcohol, and 22.7 g.of 1-4 butylene glycol were placed in a round bottomed flask equippedwith a reflux condenser and a water trap. 50 cc. of xylene was added asan entraining agent for the water of esterification. The reactionmixture was heated to reflux temperature and maintained at thattemperature until the theoretical amount of water resulting fromcomplete esterification was removed. Samples of the final product wereremoved and titrated with standard potassium hydroxide solution to checkthe acidity. The final product was found to require 0.50 mg. KOH/ g. forneutralization.

100 g. of this ester product were admixed with 8 g. (8.0%) of ethylenecarbonate and were heated for 4 hours at 160 C. After this treatment theresidual acidity was tested and 0.05 mg. KOH/ g. were needed forneutralization.

Example 11 Following the procedure outlined in Example I above a secondcomplex ester synthetic lubricant was prepared from 74 g. of adipicacid, 65.6 g. of isooctyl alcohol and 22.7 g. of 1-3 butylene glycol.

Titration of this final product showed a residual acidity equivalent to0.79. mg. KOH/ g.

g. ofthis product was admixed with 10 g. ethylene carbonate and heatedfor 4 hours at a temperature of 160 C. The resulting residual aciditywas reduced to an equivalent of 0.02 mg. KOH/ g.

Example Ill Following the procedure outlined in Example I above a thirdcomplex ester synthetic lubricant was prepared from 74 g. of adipicacid, 65.6 g. of 2-ethylhexyl alcohol and 22.7 g. of 1-3 butyleneglycol.

Titration of this final product showed a residual acidity equivalent to0.89 mg. KOH/ g.

100 g. of this productwas admixed with 12.0% ethylene carbonate andheated for 4.4 hours at 204 C. The resulting residual acidity wasreduced to an equivalent of 0.04 mg. KOH/g.

What is claimed is:

1. A process for the reduction of the residual acidity of a syntheticlubricating oil of the complex ester type which comprises reacting thecomplex ester with ethylene carbonate, said reaction occurring at atemperature within a range of between 100 C. and 250 C. for a period oftime sufficient to reduce the residual acidity of the complex ester toan equivalent of below about 0.5 mg. KOH/ g.

2. A process according to claim 1 wherein the synthetic lubricating oilhas the general formula wherein: R1 and R5 are the alkyl radicals ofmonobasic acids, R2 and R4 are the alkyl radicals from glycols, and R3is the alkyl radical from a dibasic acid.

3. A process according to claim 1 wherein the synthetic lubricating oilhas the general formula wherein:

R1 and R5 are the combining radicals of alcohols,

R2 and R4 are the alkyl radicals of dibasic acids, and R3 is the alkylradical of a glycol.

4. A process according to claim 1 wherein the synthetic lubricating oilhas the general formula R1OOCR2COOR3OOCR4 wherein:

R1 is the combining radical of aliphatic alcohols,

R2 the alkyl radical of dibasic acids,

R3 the alkyl radical of a glycol, and

R4 the alkyl radicals of a monobasic acid.

5. A process for the reduction of a residual acidity of a syntheticlubricating oil of a complex ester type which comprises reacting saidcomplex ester with from 0.5% to 20% by weight based on the weight of theester of ethylene carbonate, said reaction occurring at a temperaturewithin a range of between 100 and 300 C. for a period of time suflicientto reduce the residual acidity of the complex ester to an equivalent ofbelow about 0.5 mg. KOH/g.

6. A process for the reduction of the residual acidity of a syntheticlubricating oil of a complex ester type which comprises reacting saidcomplex ester with from 6% to 12% by Weight based on the Weight of theester of ethylene carbonate, said reaction occurring at a temperaturewithin a range of between and 250 C. for a period of time suflicient toreduce the residual acidity of the complex ester to an equivalent ofbelow about 0.5 mg. KOH/g.

7 8 7. A process according to clairn 6 wherein said synwherein: that:ester has the general formula i R1 and R5 are the combining radicals ofalcohols,

R1CO-O-Rz-O-OCRaCO--O R4OOCR5 R2 and R4 are the alkyl radicals ofdibasic acids, an wherein, 5 R3 is the alkyl radical of a glycol.

R1 and R5 are the alkyl radicals of monobasic acid,

R2 and R4 are the alkyl radicals from glycols, and References Cited Inthe file of thls patent R3 is the alkyl radical from a dibasic acid.UNITED STATES PATENTS 8. A process according to claim 6 wherein saidsyn- 1,817,898 Martin Aug. 4, 1931 thetic ester has the general formula10 2,537,890 Gearhart et a1. I an. 9, 1951

1. A PROCESS FOR THE REDUCTION OF THE RESIDUAL ACIDITY OF A SYNTHETICLUBRICATING OIL OF THE COMPLEX ESTER TYPE WHICH COMPRISES REACTING THECOMPLEX ESTER WITH ETHYLENE CARBONATE, SAID REACTION OCCURRING AT ATEMPERATURE WITHIN A RANGE OF BETWEEN 100* C. AND 250* C. FOR A PERIODOF TIME SUFFICIENT TO REDUCE THE RESIDUAL ACIDITY OF THE COMPLEX ESTERTO AN EQUIVALENT OF BELOW ABOUT 0.5 MG KOH/G.